With regard to a motorcycle such as a scooter type vehicle, in order to improve the riding comfort during high speed operation or to improve the turning performance at the time when the vehicle body needs to be tilted, it is necessary to optimize the vehicle's center of gravity. The engine is a heavy load for a motorcycle. With respect to the structure of the engine, the crankshaft is an example of a particularly heavy component.
In a scooter type vehicle, the footrest plate and the vehicle body cover are provided under the rider's seat and are arranged at the longitudinally central portion of the vehicle. In addition, the engine is disposed within the vehicle body cover. A large-sized scooter type vehicle mounted with a parallel-type (so-called “transverse”) 2-cylinder engine is conventionally known. See, for example, JP-A No. 2003-335284. This vehicle is provided as an approach to providing a vehicle that operates under a more comfortable driving environment. However, a further increase in the number of cylinders is desired.
In a large-sized scooter type vehicle, the footrest plate is often composed of a pair of right and left footrest plates. These right and left footrest plates are provided so as to extend longitudinally outside on the right and left of the vehicle body cover, respectively. In such an arrangement, the rider is seated on the seat, and steers the vehicle while taking a position with the right and left feet rested on the respective footrest plates so as to straddle the vehicle body cover.
A scooter type vehicle must be designed so as to allow the operator to rest both feet on the footrest plates without difficulty. In order to secure such a stable driving position, a scooter type vehicle is subject to limitations regarding the vertical position of the footrest plates relative to the seat as well as limitations regarding the lateral length of the vehicle body cover (the distance between the right and left footrest plates).
For example, according to JP-A No. 2003-335284, in related scooter type vehicles with a 2-cylinder engine in which cylinder chambers are arranged laterally side by side, the center axis position of the crankshaft as a heavy load is at substantially the same height as the footrest plates. Accordingly, the center of gravity of the vehicle is located at a relatively low position near the footrest position. However, depending on the vehicle's concept with regard to the turning performance or comfort, there may be cases where a relatively high center of gravity is preferred. Further, when the operating portion of the engine such as the crankshaft is disposed in the vicinity of the footrest plates as described above, this may cause the vibration of the operation portion to be readily transmitted to the footrest plates, which gives rise to the problem of how to reduce vibration acting on the operator's feet.
In addition in an internal combustion engine, vibration is generated as the piston makes reciprocating motion. In order to remove this vibration, a balancer weight such as a heavy load is rotated in synchronization with the reciprocating motion of the piston to thereby cancel out the vibration. See, for example, JP Patent No. 2860793. The balancer weight is provided on a balancer shaft that is rotatably disposed and adapted to rotate in synchronization with the crankshaft, that is, with the reciprocating motion of the piston.
However, according to JP Patent No. 2860793, the related balancer weight is mounted to a dedicated balancer shaft to which only the balancer weight is mounted. Accordingly, in addition to the balancer shaft, a gear train for rotationally driving the balancer shaft, a support member for rotatably supporting the balancer shaft, and so on are required as dedicated components, which may cause not only an increase in the number of parts but also an increase in the overall size of the internal combustion engine because a dedicated space for arranging these components must be secured within the housing.